Where data of a digital format is transmitted via a communication channel (transmission channel) in which data error (transmission error) is liable to occur, in general, a method of transmitting the data after performing a predetermined encoding on the data to be transmitted.
For example, where data is transmitted via a wireless channel, particularly, where data is transmitted via a satellite communication channel, the strength of the received signal (signal level) at the reception side becomes very low, so data error is liable to occur. Further, even in a case where data is transmitted via a communication channel using a ground wave (ground wave communication channel), the reception level becomes low at the region at the end of the service range, so data error becomes liable to occur.
Accordingly, where the data to be transmitted is encoded with a high compression rate by the high-efficient coding method, it is necessary to take strict countermeasures against data error at the step of decoding on the reception side.
For example, in the decoding on the reception side, there is a case where data other than the data in which the data error occurs is affected (data error is propagated). In such a case, it is particularly necessary to take strict countermeasures against data error.
Due to the above situation, when transmitting data via a satellite communication channel, since the carrier-to-noise ratio (C/N ratio) of the reception signal becomes small, in general, after the data of the digital format to be transmitted is encoded using a block code such as a Reed-Solomon code, it is further encoded for correction of transmission line error by convolutional coding to generate a transmission line code.
The method has been adopted of transmitting the transmission line code subjected to countermeasures against data error in this way via the communication line after modulation by QPSK modulation method etc.
On the other hand, in a ground wave communication line in which a C/N ratio of the reception signal better than that of a satellite communication line is obtained, to raise the transmission efficiency, data is frequently transmitted by modulating the transmission code by a multi-value modulation method such as a 16-QAM or 32-QAM method.
Further, recently, as a transmission line coding system combining error correction technology and modulation technology, code modulation systems are now being used. Among the code modulation systems, one using a Trellis code as the error correction code is referred to as the Trellis code modulation (TCM) system.
In general, a method using an error correction code for the transmission of data has the characteristic that when the encoding rate is made small, that is, when the ratio between the amount of information to be transmitted and the amount of information actually transmitted on the communication line is made small, the error correction capability becomes high, but the transmission efficiency (throughput) of the digital information to be transmitted is lowered, and conversely when the encoding rate is made large, the throughput becomes large, but the error correction capability becomes low.
On the other hand, transmission of data via a satellite communication line has the characteristic that the level of reception fluctuates greatly according to the gain of the reception antenna or the weather conditions.
Further, transmission of data via a ground wave communication line has the characteristic that the level of reception fluctuates according to the positional relationship between the transmission side and the reception side. That is, where the reception side is at an area near the transmission side, the level of reception becomes high, but when the reception side is at an area distant from the transmission side, the level of reception becomes low.
When transmitting data via these wireless communication channels, when the level of reception is low, the C/N ratio becomes small and data error becomes liable to occur. When the C/N ratio becomes further smaller and data error exceeding the error correction capability possessed by the transmission code is caused, the data error can no longer be corrected. That is, when subjecting the data to high-efficient coding, if the C/N ratio is degraded on the reception side, the original data can no longer be correctly reproduced.
Here, a concrete explanation will be made of the problems in transmission of data via a wireless communication channel and the countermeasures taken with respect to these problems by taking as an example a case where a television signal of digital format subjected to the high-efficient coding is transmitted.
FIG. 1 is a view showing the relationship between the carrier-to-noise ratio (C/N ratio) on the reception side and the signal-to-noise ratio (SIN ratio) of a reproduced television signal when transmitting a television signal via a wireless communication channel and the possibility of reproduction of the data.
FIG. 2 is a view showing the desirable relationship between the C/N ratio on the reception side and the S/N ratio of a reproduced television signal when transmitting a television signal via a wireless communication channel and the possibility of reproduction of the data.
Note that, the signal-to-noise ratio (S/N ratio) of the ordinates of FIG. 1 and FIG. 2 is a value determined by encoding performed on the transmission side on the data to be transmitted.
As indicated by the hatching in FIG. 1, on the reception side, in a region where a predetermined value or more of the C/N ratio of the received signal is secured (reception level II), the data of the television signal can be completely decoded from the received signal, so a good television picture can be reproduced.
On the other hand, at the reception side, in a region where the C/N ratio of the received signal becomes less than the predetermined value (reception level I), the television signal cannot be correctly decoded due to data error, so it becomes completely impossible to reproduce the television picture.
In an actual television broadcast, however, as shown in FIG. 2, it is desirable that the quality of reception of the television signal (S/N ratio) change according to the change of the C/N ratio on the reception side.
That is, desirably, in an actual television broadcast, as shown in FIG. 2, on the reception side, in a region where the signal level is high and a good C/N ratio is obtained (reception level III), a high quality television picture and sound are reproduced, while in the region where the reception level has become slightly higher than the reception level III (reception level II), a television picture and sound having a slightly lower quality are reproduced.
This is desirable because, as shown in FIG. 1, when the C/N ratio becomes less than a certain level, it is more desirable for the TV viewer that, as shown in FIG. 2, while the quality is lowered when the C/N ratio is lowered, a video signal of an extent enabling the content of the picture to be understood be reproduced at the reception side rather than it becoming completely impossible to reproduce the television signal on the reception side.
In order to realize the relationship between the C/N ratio and SIN ratio of the received signal as shown in FIG. 2, on the transmission side, it is necessary to use a hierarchical procedure in the encoding of the data to be transmitted. That is, the procedure of, at the transmission side, transmitting the data to be transmitted encoded hierarchically divided to absolutely necessary important information and additional information and, at the reception side, receiving all of the data at a high transmission speed when the signal level is high and receiving only the important information which is absolutely necessary for the reproduction at a low transmission speed when the signal level on the reception side is low, in other words, the procedure of data transmission enabling change of the throughput corresponding to the state of reception on the reception side, becomes necessary.
The present invention was made in consideration of the problems of the related art explained above and has as an object thereof to provide a signal transmitting apparatus, a signal receiving apparatus, and a method therefore whereby, when for example transmitting a television signal in a digital format, a good television picture and sound can be provided when the signal level is high on the reception side and, at the same time, a certain degree of quality of television picture and sound can be provided even in a case when the signal level is low, i.e., so-called "graceful degradation" can be realized.
Another object of the present invention is to provide a signal transmitting apparatus, a signal receiving apparatus, and a method thereof which can realize the above-mentioned relationship between the C/N ratio of the received signal and the S/N ratio of the reproduced data, that is, a relationship where when the C/N ratio of the received signal on the reception side become less than a certain value, the S/N ratio of the reproduced data is lowered according to the C/N ratio.
Still another object of the present invention is to provide a signal transmitting apparatus, a signal receiving apparatus, and a method thereof which can perform data transmission with a throughput in accordance with the state of reception, that is, the C/N ratio of the received signal.